1. Field of the Invention
This invention relates to character generators and, more particularly, to an electronic character generator which reproduces selected characters on the screen of a CRT by connecting variable length, constant intensity strokes in predetermined patterns.
2. Description of the Prior Art
A character generator having a sixty cycle per second refresh rate is highly desirable since a refresh rate of only thirty times per second often produces visible flickering, particularly in well illuminated rooms. In a CRT display having the capacity to display 6144 characters, a sixty cycle per second refresh rate requires that the character generator be able to form the most complex character in its repertoire in a 2.5 microsecond interval. The attainment of this very rapid character generation rate typically limits the complexity of the characters to block style shapes formed from vertically, horizontally and diagonally oriented strokes.
Since block style characters are often poorly defined and difficult to read, several designers have gone to great lengths to create character generators having the capability of forming accurate reproductions of more conventional and readable print styles. Meeting this design goal generally necessitates use of a slow 30 cycle per second refresh rate that provides a 5 microsecond time interval in which to form these geometrically more sophisticated characters.
The net result of designers' efforts to provide more accurate reproductions of printed characters is that the cost of character generators has remained high and as a result of the slow 30 cycle per second refresh rate the CRT displays must be placed in a room having relatively dim lighting to minimize CRT screen flicker.
U.S. Pat. No. 3,810,165 (Rosenthau) discloses a character generator which forms characters from vertical, horizontal or diagonal strokes of equal length. This system is limited to using equal length strokes since it is not capable of providing the electron beam intensity compensation necessary when variable length strokes are used to form characters. This system utilizes large numbers of stored digital bits to represent a single character since a three bit word represents either the horizontal or vertical component of a stroke while an additional sign bit is required to define either a positive or negative axis orientation for the respective X or Y magnitude component.
The Rosenthau character generator is limited to moving the CRT electron beam in any one of only eight possible directions from a single point. In this manner it generates a horizontally, vertically or diagonally oriented unit length stroke.
U.S. Pat. No. 3,755,805 (Dandrel) discloses a character generator which is capable of producing strokes of either one or two units of length. This system fails to provide any intensity compensation so that a two unit stroke would be more dimly illuminated than a one unit stroke.
This system like the Rosenthau system is limited to moving the electron beam in only one of eight possible directions.
Another prior art character generator is disclosed in U.S. Pat. No. 3,786,482 (Puckett). This apparatus is far more sophisticated than either of the two previously described character generators since it has the capability of causing the electron beam to move in any one of thirty-one possible directions from a given starting point. The refresh rate of this system is severely limited in that a comparatively lengthy time interval is required to generate a single character. While this system can accurately reproduce complex characters it requires substantial memory capacity since over fifty separate strokes are used to generate several of these complex characters and thirteen bits are required to describe each stroke. This system is further limited in that it can generate a stroke having only a single length.
Other less relevant prior art is shown and described in the following U.S. Pat. Nos.: 3,603,967 (Hauerbach); 3,643,251 (Kolb et al.); 3,671,957 (Kegelman et al.) and 3,800,183 (Halio).